The production of cementitious members usually involves the incorporation of steel reinforcement. The reinforcement may be present as structural reinforcement due to the designed use of the member in which it is included, or the steel may simply be present to allow for a member (such as a curtain wall panel) to be stripped from its mold without cracking. The inclusion of steel reinforcement requires a minimal amount of concrete cover to reduce the potential for steel corrosion.
The typical production of pre-cast or tilt-up wall panels involves the casting of concrete into horizontally positioned forms out of which the hardened concrete piece is lifted. During this lifting or “stripping” of the forms, the concrete undergoes significant flexural and tensile stresses. To keep these wall panels from potentially cracking and breaking, continuous steel reinforcement is incorporated into the cast piece. ACI 318 section 7.7 provides guidelines for the amount of concrete cover (in terms of depth) which is required to cover the steel reinforcement so that the steel is protected from weather and other effects.
Depending upon the exposure of the concrete element and the type (size) of reinforcement used, a certain amount of cover is required. In some instances this can require that the thickness of the wall panel be greater than 4 inches. These wall panels are not structural pieces but simply architectural. The thickness of the pieces creates a heavier, less mobile and more expensive piece. This weight is taken into account when designing the support members for the building. The weight also limits the number of members that can be shipped at one time due to load restrictions on most roadways.
It would be desirable to produce a thinner, lighter panel that would allow for smaller dead loads on the building and increased shipping efficiencies. To withstand the stripping stresses as well as wind and other factors once in place, the thinner panel would need to have higher strength than the conventional, thicker panel. In the same way that these thinner panels (wall elements) use steel reinforcement and require a minimum of concrete cover, structural elements (both pre-cast and cast in place) will have similar requirements.
U.S. Pat. No. 4,559,881 discloses a concrete panel comprising: (a) Portland cement; (b) fly ash; (c) fine aggregate; (d) gravel (e) melamine superplasticizing water-reducing admixture; (f) and steel fibers.
U.S. Pat. No. 4,780,141 discloses a cementitious composite mixture which comprises: (a) Portland cement; (b) chemically reactive silica particles; (c) inorganic oxide particles; (d) aggregate particles; (e) metal fibers; (f) superplasticizer; and (g) water.
JP 9631029 discloses a concrete mixture composed of: (a) cement; (b) coarse aggregate; (c) fine aggregate; (d) steel fiber; (e) fly ash; (f) a high performance AE water reducing agent of polycarboxylic acid-base or napthalene-base; and (g) a thickening agent mainly containing methyl cellulose or a water-soluble polysaccharide.
What is needed in the industry is a cementitious composition that allows for lighter and stronger corrosion resistant members. This can be accomplished by the replacement of steel with structural synthetic fibers which will remove the potential for corrosion and will significantly increase the compressive and flexural strength, particularly at ages less than 24 hours, while lowering the amount of cementitious composition required.